The present invention generally relates to a phase inverter, and more particularly to a phase inverter which receives a radio frequency signal and derives two radio frequency signals having a phase difference of 180.degree. therefrom. Further, the present invention is directed to a push-pull amplifier using such a phase inverter.
Conventionally, it is known to form a phase inverter by the use of a transformer, a capacitor or an inductor. Such a phase inverter is suitable for a low-frequency range. In other words, the use of a transformer, a capacitor or an inductor is not suitable for a very high frequency range in the order of tens of giga-hertz or higher because of the presence of stray capacitance. From this reason, it is necessary to use a distributed element circuit such as a coupled line network in the range of very high frequencies (microwave range).
Referring to FIG. 1, there is illustrated a conventional phase inverter using parallel coupled lines. Ring-shaped transmission lines 2 and 3 formed of a conductive material are arranged on a dielectric substrate 1 so that two networks each composed of two .lambda./4 parallel coupled lines are formed (.lambda. is the wavelength of a signal applied to the transmission line 3). The transmission line 2 has terminals 4.sub.2, 4.sub.3 and 4.sub.4, and the transmission line 3 has a terminal 4.sub.1. Two adjacent terminals among the terminals 4.sub.1, 4.sub.2, 4.sub.3 and 4.sub.4 have a phase difference of 90.degree.. A signal applied to the terminal 4.sub.1 is equally distributed to the terminals 4.sub.2 and 4.sub.4. A signal output from the terminal 4.sub.2 has the +90.degree. phase difference with respect to the input signal, and a signal output from the terminal 4.sub.4 has the -90.degree. phase difference with respect to the input signal. Thus, the signals having a phase difference of 180.degree. are output through the terminals 4.sub.2 and 4.sub.4. In this case, the signals output from the terminals 4.sub.2 and 4.sub.4 are combined together at the terminal 4.sub.3. However, since the signals at the terminals 4.sub.2 and 4.sub.4 are 180.degree. out of phase, no signal is output from the terminal 4.sub.3.
FIG. 2A is a graph of amplitude vs. normalized frequency characteristics of the conventional phase inverter shown in FIG. 1, and FIG. 2B is a graph of phase vs. normalized frequency characteristics thereof. A curve I is a characteristic of a signal passing from the terminal 4.sub.1 to the terminal 4.sub.2. A curve II is a characteristic of a signal passing from the terminal 4.sub.1 to the terminal 4.sub.4. It can be seen from FIG. 2A that the amplitude characteristic of the signal passing from the terminal 4.sub.1 to the terminal 4.sub.2 is approximately flat in the .+-.0.4f.sub.0 range where f.sub.0 is the center frequency of this signal (equal to 12.5 GHz, for example). On the other hand, the amplitude characteristic of the signal passing from the terminal 4.sub.1 to the terminal 4.sub.4 is approximately flat only in the .+-.0.2f.sub.0 range. Such a range is narrow and insufficient in view of practical board-band use.
It can be seen from FIG. 2B that the phase characteristics of the signal passing from the terminal 4.sub.1 to the terminal 4.sub.2 and the signal passing from the terminal 4.sub.1 to the terminal 4.sub.4 have a phase difference of 180.degree. in a range of .+-.0.4f.sub.0. From the above-mentioned reasons, it is desired to provide a phase inverter in which the amplitude and phase characteristics of two 180.degree. out-of-phase output signals are both flat in a wider frequency range.